1. Field of the Invention
The present invention relates to a control device configured to control a light source element and an image forming apparatus with the control device built therein.
2. Description of the Related Art
An image forming apparatus such as a copying machine, a facsimile device, a scanner device or a multi functional peripheral (MFP: Multi Functional Peripheral) with their functions typically comprises a linear lamp configured to irradiate light to a document and a CCD sensor (Charged Coupled Device Sensor) configured to receive reflected light from the document. The image forming apparatus generates image data corresponding to an image of the document based on the reflected light received by the CCD sensor.
The image forming apparatus forms a latent image on a circumferential surface of a rotating photoreceptor based on the image data. The image forming apparatus thereafter supplies toner to the latent image to form a toner image. The toner image is transcribed to a sheet conveyed in the image forming apparatus. The toner image is thereafter heated and fixed to the sheet.
The lamp has to emit light of which quantity is a given value or higher for accurate reading of the image with the CCD sensor. The light quantity of the lamp generally depends on amplitude of an applied voltage. A larger applied voltage results in a larger lamp light quantity and shortens a period until the light quantity reaches the given value. On the other hand, a smaller applied voltage results in a smaller lamp light quantity and lengthens the period until the light quantity of the lamp reaches the given value.
Increase in the applied voltage grows a heat value of the lamp. Consequently, the temperature of the lamp more largely rises up.
FIG. 6 is a schematic perspective view of a linear lamp typically used in an image forming apparatus. The lamp is now described with reference to FIG. 6.
The lamp L comprises a cylindrical light emitter E configured to emit light toward a document and support members F configured to support both ends of the light emitter E, respectively. The support member F is mounted, for example, directly or indirectly to a housing of the image forming apparatus. The support member F is typically molded from resin.
In terms of cost reduction in manufacturing image forming apparatuses, the support member F is often molded from less expensive resin. Many types of the inexpensive resin are less heat resistant.
FIG. 7A is a graph schematically showing a relationship between an application period of a voltage applied to the lamp L and the applied voltage. FIG. 7B is a graph schematically showing a relationship between the application period of the voltage applied to the lamp L and an amount of a reflected light from a document. FIG. 7C is a graph schematically showing a relationship between the application period of the voltage applied to the lamp L and a temperature of the lamp L.
A light quantity has to be a given value or higher for accurate reading of the image with the CCD sensor. In order to quickly obtain the light quantity that is a given value or higher, as shown in FIG. 7A, a higher constant voltage is applied to the lamp L. Consequently, an amount of light irradiated from the lamp L increases. As shown in FIG. 7B, as the lamp L increases the light quantity, the amount of the reflected light from the document also goes up. The increase in the light quantity of the lamp L causes a temperature rise of the lamp L, as shown in FIG. 7C. The temperature of the lamp L sometimes increases beyond a heatproof temperature (shown with a dotted line in FIG. 7C) of resin used for the support member F.
FIG. 8A is a graph schematically showing a relationship between an application period of a voltage applied to the lamp L and the applied voltage in an improved image forming apparatus. FIG. 8B is a graph schematically showing a relationship between the application period between the voltage applied to the lamp L and an amount of reflected light from a document in the improved image forming apparatus. FIG. 8C is a graph schematically showing a relationship between the application period of the voltage applied to the lamp L and a temperature of the lamp L in the improved image forming apparatus.
As shown in FIG. 8A, a higher voltage Va is applied to the lamp L in order to quickly obtain a light quantity that is a given value or higher. As the lamp L increases the light quantity, as shown in FIG. 8B, the amount of the reflected light from the document goes up and exceeds a given value of the light quantity (shown with a dotted line in FIG. 8B), so that the CCD sensor accurately reads the image. When the amount of the reflected light from the document exceeds the given value of the light quantity, as shown in FIG. 8A, the applied voltage to the lamp L is switched to a voltage Vb lower than the voltage Va. The voltage Vb is experimentally set forth so that the temperature of the lamp L does not exceed a heatproof temperature of resin used for the support member F. The temperature rise of the lamp L moderates as a result of switching from the voltage Va to the voltage Vb. Accordingly, the lamp L is less likely to increase the temperature over the heatproof temperature of the resin used for the support member F (cf. FIG. 8C).
The control described with reference to FIGS. 8A to 8C sometimes does not moderate the temperature rise of the lamp L. For example, when it takes so long that the lamp L starts to emit after the application of voltage Va, when growth rate of the amount of the light irradiated from the lamp L is so low, or when an element (for example, the foregoing CCD sensor) configured to detect the reflected light less accurately detects the amount of the reflected light, the lamp L controlled as described above potentially causes an excessive temperature rise.
A further improved image forming apparatus comprises a lamp configured to change a timing for starting emission according to a period how long the lamp is placed under a dark environment, an emission circuit configured to light the lamp, a detection element configured to detect a light quantity of the lamp and a control element configured to control the emission circuit. If the light quantity of the lamp after lapse of a predetermined period from the start of the lamp emission falls below a given value, the lighting circuit temporarily turns off the lamp under the control of the control element. The emission circuit thereafter re-lights the lamp under the control of the control element (retry control). The retry control of the control element is likely to prevent erroneous operation of the emission circuit resulting from the delay in the start timing of the lamp emission.
FIG. 9A is a graph schematically showing a relationship between an application period of a voltage applied to the lamp and the applied voltage in the further improved image forming apparatus. FIG. 9B is a graph schematically showing a relationship between the application period of the voltage applied to the lamp and an amount of reflected light from a document in the further improved image forming apparatus. FIG. 9C is a graph schematically showing a relationship between the application period of the voltage applied to the lamp and a temperature of the lamp in the further improved image forming apparatus.
As shown in FIGS. 9A to 9C, the foregoing retry control is performed while the light quantity of the lamp increases. Consequently, a period required for detecting the amount of the reflected light from the document becomes longer. A temperature rise of the lamp moderates independently from variations in the lamp-related element or changes in a peripheral environment of the lamp. As shown in FIG. 9A, however, the temperature rise of the lamp potentially continues because a voltage as great as the voltage before the retry control is also applied to the lamp after the retry control. Consequently, the lamp temperature sometimes exceeds the heatproof temperature of the support member.